1. Field of the Invention
The present invention relates to a control system for an automatic transmission.
2. Related Art
In the automatic transmission of the prior art, the torque generated by the engine is transmitted through a fluid transmission unit, such as a torque converter, to a speed change unit. Between the fluid transmission unit and the speed change unit, is arranged a first clutch acting as an input clutch, which can be applied/released for change between neutral and a forward running range.
In such an automatic transmission, when the range is changed from neutral to forward running (hereinafter "N-D change") by operation of a speed selector such as a shift lever, the N-D change can controlled with smooth application of the first clutch by gradually boosting the oil pressure of the hydraulic servo of the first clutch. Moreover, when the forward running range is selected with the vehicle at a stop, a neutral control can bring the first clutch into a predetermined released state by reducing the oil pressure of the hydraulic servo of the first clutch (as disclosed in U.S. Pat. No. 4,850,251). For this purpose, the hydraulic control system has a control valve, responsive to a signal pressure, for generating a control oil pressure and has a change-over valve for selectively feeding either the control oil pressure or a line pressure (also referred to as "forward running range pressure") to the hydraulic servo. For control of the hydraulic servo, the control system for the automatic transmission switches the change-over valve to connect the control valve and the hydraulic servo to thereby feed the control oil pressure from the control valve to the hydraulic servo. As a result, the first clutch can be placed in a predetermined engaging state corresponding to the control oil pressure. Moreover, the control system switches the change-over valve, when the oil pressure of the hydraulic servo need not be controlled, to feed the line pressure (or "forward running range pressure") to the hydraulic servo. As a result, the first clutch can be brought into the engaged state.
In such a hydraulic control unit, if, for some reason, the control oil pressure is not produced by the control valve the result is a failed state. In this case, the change-over valve can be switched to feed the line pressure to the hydraulic servo to thereby apply the first clutch. Thus, the control system has a fail-safe feature for releasing the N-D control or the neutral control so that the first clutch can be applied to start the vehicle, where a failure occurs when the vehicle is to be started during the N-D change control or neutral control.
However, in the above-described control system a shift shock may occur as the first clutch is applied. For example, a state of failure may be decided by the timing operation of a guard timer. In case the vehicle is to be started during the N-D change control, the timing operation of the guard timer is started when a N-D change is indicated by operation of the shift selector. If the engagement of the first clutch is not ended by expiration of the timed period, it is decided that a failure has occurred.
On the other hand, in case the vehicle is to be started during neutral control, the driver will remove his foot from the brake pedal and depress the accelerator pedal. When these starting operations are performed, the timing operation by the guard timer is started. If the engagement of the first clutch is not completed by the end of the timing period, it is decided that a failure has occurred.
When the accelerator pedal is depressed simultaneously with the N-D change, for the purpose of starting the vehicle moving during the N-D change control, the engine R.P.M. rises by the time it is decided that a failure has occurred. As a result, engaging shock will occur if, responsive to a decision that a failure has occurred, the N-D change control is released to apply the first clutch.
Likewise, if the accelerator pedal is depressed simultaneously with release of the brake pedal so as to start the vehicle moving during neutral control, the engine R.P.M. rises by the time the occurrence of a failure is decided. As a result, an engaging shock will occur, if the occurrence of a failure is decided and the neutral control is accordingly released to apply the first clutch.